Fluid ejection devices in inkjet printers provide drop-on-demand ejection of fluid drops. Inkjet printers produce images by ejecting ink drops through a plurality of nozzles onto a print medium, such as a sheet of paper. The nozzles are typically arranged in one or more arrays, such that properly sequenced ejection of ink drops from the nozzles causes characters or other images to be printed on the print medium as the printhead and the print medium move relative to each other. In a specific example, a thermal inkjet printhead ejects drops from a nozzle by passing electrical current through a heating element to generale heat and vaporize a small portion of the fluid within a firing chamber. Some of the fluid displaced by the vapor bubble is ejected from the nozzle. In another example, a piezoelectric inkjet printhead uses a piezoelectric material actuator to generate pressure pulses that force ink drops out of a nozzle.
Although inkjet printers provide high print quality at reasonable cost, their continued improvement depends in part on overcoming various operational challenges. For example, the release of air bubbles from the ink during printing can cause problems such as ink flow blockage, insufficient pressure to eject drops, and mis-directed drops. Pigment-ink vehicle separation (PIVS) is another problem that can occur when using pigment-based inks. PIVS is typically a result of water evaporation from ink in the nozzle area and pigment concentration depletion in ink near the nozzle area due to a higher affinity of pigment to water. During periods of storage or non-use, pigment particles can also settle or crash out of the ink vehicle which can impede or block ink flow to the firing chambers and nozzles in the printhead. Other factors related to “decap”, such as evaporation of water or solvent can cause PIVS and viscous ink plug formation. Decap is the amount of time inkjet nozzles can remain uncapped and exposed to ambient environments without causing degradation in the ejected ink drops. Effects of decap can alter drop trajectories, velocities, shapes and colors, all of which can negatively impact the print quality of an inkjet printer.